Offshore oil production requires that subsea wells be drilled and that equipment such as Christmas tree manifolds, oil flow lines, gas injection lines and water injection lines be installed between the wellheads and the processing facilities. These processing facilities may be located on a vessel, platform, or even on land.
Christmas tree manifolds are assemblies of connectors and valves installed at the top of an oil wellhead and are used to block (open and close) and route the flow of fluids produced or injected.
There are two types of wet Christmas tree (WCT) manifolds, depending on how the valves are arranged with regard to the production column hanger inside the well: horizontal and vertical (which is conventional). In some cases, mainly with a vertical wet Christmas tree manifold, additional equipment known as a production adaptor base (PAB) is employed between the WCT and the wellhead. The production adaptor base is mainly intended to support the oil-production and gas-injection lines, as well as the production column. In the latter case, the tubing hanger is attached to the production adaptor base rather than to the WCT. Flow-lines supported by the production adaptor base are connected to the wet Christmas tree, which is in turn connected to the well bottom by the production column.
Inasmuch as producing wells are located at some distance from the processing facilities, sufficient pressure must be maintained at the wellhead so that fluids can flow to the facilities at a reasonable flow rate so as to make the project economically feasible.
Various lifting methods are used to boost flow from the wellhead to the processing facilities. One such method uses pumps, for example centrifugal pumps known as electrical submersible pumps (ESP). Installed at the bottom of oil-producing wells, they are generally driven by an electric motor.
Once the ESP, production column and wet Christmas tree have been installed at the well, maintenance performed on the ESP increases operating costs and risks, involving the equipment as well as the environment.
From time to time it becomes necessary to perform maintenance or repairs on the ESP installed in the well. Removal of an ESP from the subsea well requires an interruption (loss) of oil production for several days, along with the use of additional equipment and vessels equipped with a rig so that the ESP and production column can be removed, resulting in higher production costs.
U.S. Pat. No. 6,497,287 describes how an electrical submersible pump is typically used in the production of offshore oil.
Furthermore, U.S. Pat. No. 5,280,766 shows that an ESP can be installed or recovered from the well bottom without removing the subsea Christmas tree, through flexible steel tubing known as “coil tubing” or “flexi-tubo.”
Based on Applicant's Brazilian Application PI 0301255-7, herein incorporated in full as a reference, it is known that it is possible to use a pumping module installed and retrieved by cable from a nonspecialized vessel. Such a module is comprised of a closed tubular body and a hydraulic connector, whereby said connector is coupled to the mandrel of an intermediate flow inlet (IFI), thus establishing a hydraulic communication between the pumping module and the cavities (holes) in said mandrel, i.e., for pump suction and discharge. Such an application has the disadvantage of requiring changes in the PAB-WCT assembly that alter the manufacturing standard by increasing the weight, size and cost of said assembly. Moreover, when the pumping module is mounted vertically, its upper end extends above the WCT, hampering light activity at these wells due to the need to remove this module to avoid the risk that the completing riser-blow out preventer (BOP) will strike the module during connection for reentry into the well. Another drawback of this system is that it cannot be easily applied to existing wells owing to the need to change the production adaptor base (PAB), in other words, re-completing the well with removal of the production column is required.
U.S. Pat. Nos. 4,900,433 and 6,036,749 show a vertical oil separator system with a pump similar to an ESP, wherein a separator-pump assembly is installed in a dummy well built for the sole purpose of holding the separation-pumping assembly and whose pump is similar to an ESP with rather slender geometry, i.e., thin and long, designed preferably to operate in the vertical position.
Moreover, U.S. Pat. No. 6,688,392 shows that it is possible to install a motor pump assembly, similar to an ESP, hydraulically linked to a dummy well. Nevertheless, this solution presents a number of drawbacks: the dummy well is hydraulically linked to the flow from the oil well, and operates under oil pressure. When the liner of the dummy well corrodes, maintenance becomes difficult because the liner is buried in the seabed and cannot be recovered. Although there is a reference to a bypass, the proposed geometry has no provision or possibility for a pig (i.e., a flow line cleaning device) to pass, a basic necessity to ensure oil well flow. Nor is another lifting method described to maintain production continuity in case the proposed pumping system fails. The dummy well is of conventional construction, i.e., drilled and cemented. In addition, the pump installation method requires a vessel with a coil tubing unit. Connection of the proposed pumping system has no metal-metal seal.
Despite the above developments, there is still a need in the state of art for a cable-based system and method for installing and recovering a centrifugal pumping module that does not have the drawbacks described above.